In GSM like systems, the output power in a transmit mode of the transmitter is controlled by a power amplifier (PA). Typically, power amplifiers are controlled by controlling the current drawn by the power amplifier. This current is proportional to the output power. Alternatively, other parameters proportional to the output power can be regulated.
By sensing the current supplied to the power amplifier, and feeding it back to an error amplifier for feedback to a control input of the power amplifier, the biasing of the power amplifier can be changed in order to regulate the output of the power amplifier. Typically a filter would be used in the feedback loop in order to limit the amount of noise introduced into the system.
However, the typical transfer function of the power amplifier, or its gain (control signal to current consumption or output power) is not constant. When a mismatch occurs at the antenna, the load on the power amplifier is changed. This also means that the output power error gets bigger for a given voltage standing wave ratio (VSWR) level. As mentioned above, typically the DC current supplied to the power amplifier is regulated, but this means that the relationship between power out and DC current is only known if the load at the output at the PA is constant. As the load of the antenna varies, the output power from the PA varies differently for different phases at high VSWR. For example, at VSWR 8:1 for the different phases of the output power could vary by as much as +/−9 dB if there were not other limitations that limit the power output. This can be shown to be the case due to the expression P=I2RL/2, where in real terms RL varies from 6.25 ohms to 400 ohms at VSWR8:1. Accordingly, if better control is required, then information about the output power or the reflected power has to be used instead of the total DC power supplied to the power amplifier.